The invention described in this patent application relates to nondestructive measurement and mapping of materials, particularly using microwaves in the measurement of carrier concentration and mobility and sheet resistance in semiconductor wafers and flat panel displays.
Existing instruments that employ microwaves in the measurement of sheet resistance in GaAs wafers employ an X-band waveguide configuration with a characteristic impedance of 300 ohms which does not permit accurate measurement of wafers with thin cap layers. Such instruments are discussed in U.S. Pat. No. 4,605,893, to Norman Braslau. One disadvantage of these instruments is an inability to separately measure the properties of multiple conducting layers in wafers that contain such layers, e.g. a high-electron-mobility transistor (HEMT) wafer, which contains a two-dimensional (2D) channel layer and a cap layer. Therefore, one must measure the sheet resistance and mobility at temperatures lower than 300 degrees Kelvin, such as at 77 degrees Kelvin, in such existing systems in order to increase the channel-layer mobility relative to the conductance of the cap layer. These instruments also require involved calibration procedures.
Destructive techniques for measurement of sheet resistance and mobility are also known. However, such techniques necessarily damage the wafers or other material being measured.
A device for nondestructive measurement of mobility and carrier concentration in sheet materials includes a microwave source, a circular waveguide configured to transmit microwaves received from the microwave source to a conductive sheet material, such as a semiconductor wafer or flat panel display, a first detector for receiving the forward microwave power, a second detector for detecting the microwave power reflected from the material, and a third detector for detecting the Hall effect power. A circular waveguide, carrying only the TE11 mode, is terminated by the wafer behind which, at a distance of xc2xc wavelength, a short is located. Perpendicular to the plane of the wafer (and along the axis of the waveguide), a variable magnetic field is applied. In this configuration, a given incident TE11 wave will cause two reflected waves. One is the ordinary reflected wave in the same polarization as the incident one. This is used to measure sheet resistance. The other reflected wave is caused by the Hall effect. Its polarization is perpendicular to the former wave and this can be separately detected with a properly configured probe.
A device for measurement of mobility and carrier concentration in conductive sheet materials includes a microwave source; a circular waveguide positioned to receive microwave radiation from the microwave source; a mount adapted to position a sheet material item at a measurement location to receive microwave radiation transmitted from the circular waveguide; a magnet positioned to induce a magnetic field at the measurement location; a first detector positioned to detect the power of the microwave radiation source; a second detector positioned to detect the power of the microwave radiation reflected from a sheet material item in the measurement location; and a third detector positioned to detect a Hall effect microwave power. A fourth detector may be provided to detect magnetic field strength at the measurement location.
A method for measurement of mobility and carrier concentration in conductive sheet materials includes generating microwave radiation; transmitting only the TE11 mode of the generated microwave radiation successively to a conductive short and to a sheet material sample; during the step of transmission, applying a magnetic field of selected intensity; detecting microwave field strength at a forward position; detecting microwave field strength reflected from each of the conductive short and the sample; detecting Hall effect microwave field strength; and, based on the detected microwave field strengths, calculating values for mobility and carrier concentration.